Aluminide coating removal method

ABSTRACT

An improved method for removing an aluminide coating from an article includes a sequential treatment of an aqueous mixed acid solution of nitric and phosphoric acids, an alkaline permanganate aqueous solution and then the mixed acid solution repeated. In one form in which the article has been exposed to air and products of fuel combustion at elevated temperatures, the method combines a mechanical pretreatment with the chemical stripping operation. An optional final mechanical removal of residue can be included. The mechanical pretreatment includes the use of angular metallic particles, such as steel, which are soluble in a subsequently applied solution, such as the mixed acids.

United States Patent Grisik et al.

[ ALUMINIDE COATING REMOVAL METHOD [75] Inventors: John J. Grisik, Cincinnati, Ohio;

Ellis J. Airola, Ipswich, Mass.

[73] Assignee: General Electric Company,

Cincinnati, Ohio [22] Filed: Sept. 5, 1972 [21] Appl. No.: 286,196

[52] US. Cl 134/3, 134/7, 134/26, 1.34/28, 134/41, 134/42, 156/22, 252/79.2, 252/79.5, 252/103, 252/142 [51] Int. Cl. 1308b 3/08, C23g l/02, C23g 1/14 [58] Field of Search 134/3, 27, 28, 41, 26, 134/42, 7; 204/1415; 252/79.2, 101, 103, 142

[56] References Cited UNITED STATES PATENTS 3,000,829 9/1961 Arden 252/103 3,085,917 4/1963 Netzler et al. 134/27 3,216,857 11/1965 Duvall 134/3 3,457,107 7/1969 Mickelson et al. 134/28 Sept. 3, 1974 3,607,398 9/1971 Lucas 252/142 X 3,622,391 11/1971 Baldi 134/3 Primary Examiner-Joseph Scovronek Assistant Examiner-Barry I. Hollander Attorney, Agent, or Firm-Derek P. Lawrence [5 7] ABSTRACT An improved method for removing an aluminide coating from an article includes a sequential treatment of an aqueous mixed acid solution of nitric and phosphoric acids, an alkaline permanganate aqueous solution and then the mixed acid solution repeated. In one form in which the article has been exposed to air and products of fuel combustion at elevated temperatures, the method combines a mechanical pretreatment with the chemical stripping operation. An optional final mechanical removal of residue can be included. The mechanical pretreatment includes the use of angular metallic particles, such as steel, which are soluble in a subsequently applied solution, such as the mixed acids.

6 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relates to the removal of coatings from metallic surfaces and, more particularly, to an improved method for removing from a metallic surface an aluminide coating, in one particular form, one which has been subjected to high temperature operating conditions. The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the US. Department of the Air Force.

The quest for more efficient gas turbine engines has led to the wide spread use of increasingly intricate and expensive air cooled components in hot sections of the engine. The cost of such components has increased as a result of the more sophisticated design as well as the advanced alloys employed. Consequently a corresponding increase in service life is required to maintain the cost per operating hour at an acceptable level. To a great degree the extension of the service life has been accomplished, and it is not uncommon for such components to have an expected life time of at least about 10,000 hours. In order to achieve the expected life time, it is necessary to provide environmental protection for those components operating at high temperatures in air and in the presence of corrosive products and products of fuel combustion.

At present, the most common, reliable and least expensive method of providing the necessary protection is through the use of an aluminide diffusion coating. While the coating provides adequate protection, it is normally life-limited to the range of up to about 5000 hours. Therefore, the necessity arises to renew the protection provided by the coating several times during the life of the part. The removal of the coating is made more difficult since the coating has been designed and developed to resist attack. A significant problem, therefore, is one of removing the coating without affecting the structural material of the article.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide an improved method for removing all types of aluminide coatings while maintaining the integrity of and avoiding intergranular attack on the coated article.

Another object is to provide, for use with articles having cooling passages, such an improved method which avoids cooling hole blockage and dimensional changes.

These and other objects and advantages will be more clearly understood from the following detailed description and examples which are intended to be typical and representative of rather than limiting on the scope of the present invention.

Briefly, the method of the present invention, in one form, comprises subjecting the coating to an aqueous mixed acid solution consisting essentially of greater than 25 percent to less than 75 percent by volume nitric acid of about 70 weight percent HNOa and greater than 25 percent to less than 75 percent by volume phosphoric acid of about 85 weight percent H PO After rinsing the mixed acid solution from the coating, it is subjected to an alkaline permanganate solution consisting essentially of, by weight, about 8 ll percent sodium hydroxide, about 8 11 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities. The alkaline permanganate solution is then rinsed from the coating and the coating is again subjected to the mixed acid solution.

The method of the present invention, in a form in which it is used for removing an aluminide coating from an article which has been exposed to air and products of fuel combustion at elevated temperatures, provides a mechanical pretreatment involving abrading the coating to a surface finish of no greater than about rms (root mean square) with angular particles, preferably metallic, which are soluble in a subsequently applied liquid. In another form, after sequential treatment with the aqueous solutions and after rinsing, the surface on which the coating existed is mechanically treated to remove any chemical residue.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention provides an improved balance between speed and safety for stripping aluminide coatings from a variety of high temperature operating superalloys including those based on nickel as well as those based on cobalt. Through the alternate use of acid solutions and alkaline solutions, the process avoids termination of aluminide coating removal due to passivation yet avoids detrimental attack on the base material which was coated.

The method of the present invention is useful in removing aluminide coatings which have been exposed to elevated temperature operating conditions in air and in the presence of products of combustion. Such coatings are particularly difiicult to remove because of an oxide layer which is created under such operating conditions.

In the evaluation of the present invention. a variety of known and development solutions were tested. Commercial alkaline nickel strippers, for example containing ammonium salts, were initially effective in attacking the nickel. However, after a short time the attack stopped due to passivation. Other products including mixtures of organic and inorganic compounds, which included alkaline sodium cyanide mixtures, were tested. The alkali attacked the aluminum while the cyanide constituents attacked the nickel. Passivation did not occur since both the aluminum and, in the case of the nickel base alloy, the nickel are attacked concurrently. However, the method provided little or no stripping action on articles which had an oxide layer or layers due to operating exposure. Other solutions containing combinations of nitric and hydrofluoric acids were evaluated in connection with mixtures including sodium cyanide. Although the combination worked reasonably well, there is a serious safety problem using acid and cyanide salts in the same operation, because of the danger of forming the poisonous hydrogen cyanide gas, and possible intergranular attack of the base alloy. In general, stripping of articles which had been oxidized in air in the presence of products of fuel combustion for periods of 100 hours or more using known solutions resulted in very sluggish or incomplete aluminide coating removal, particularly in the stripping of aluminides from cobalt base alloys.

In the practice of the method of the present invention, two principal aqueous chemical solutions are provided. The first is an aqueous mixed acid solution of mixed nitric and phosphoric acid. The amounts of acids included are equivalent to those which consist essentially of greater than 25 to less than 75, and preferably 45 55, volume percent nitric acid of about 70 weight percent HNO and greater than 25 to less than 75, and preferably 45 55, volume percent phosphoric acid of about 85 weight percent H PO The second solution is an alkaline permanganate solution consisting essen tially of, by weight, about 8 ll percent sodium hydroxide, about 8 11 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities.

. In an initial evaluation of the present invention, a series of tests were conducted using the above identified type of mixed acid solution to establish the interrelationship between its components. The specimens tested were of a nickel base superalloy of a type described in US. Pat. No. 3,027,254 issued Mar. 27, 1962 and sometimes referred to as SEL alloy. The coating was an aluminide which had been applied by a pack diffusion method and which had been aged for 2 hours in hydrogen at about lO0C. Typical of such tests are those summarized in the following table.

tion in a jet engine, for example, at least about 100 hours, to remove all of the coating. This was due to the presence of oxides and products of fuel combustion on or in the coating surface. For example, a nickel base superalloy in the form of a turbine blade and including an aluminide coating applied through a pack diffusion method was evaluated for coating removal after the turbine blade had been used in a jet engine. The blade was immersed in the 1:1 HNO /H PO mixed acid solution at 80C for 2 hours after which it was rinsed and immersed in the alkaline permanganate solution consisting essentially of, by weight, 10.9 percent sodium hydroxide, 10.9 percent anhydrous sodium carbonate, 5.5 percent potassium permanganate with the balance essentially water and incidental impurities at a temperature of 80C for 1 hour. After rinsing, it was again immersed in the above identified mixed acid solution for 2 hours. After rinsing it was mechanically abraded with a number 1250 grit silicon dioxide, suspended in water, to remove a chemical residue or smut which remained on the surface. Examination of the article surface after treatment showed such surface to be particularly rough in the ar ea on which heavy oxides had been generated shown by the data of the table, the test of example 2 within the range of the amount of mixed acid solution used in the practice of the method of the present invention provided removal of all of the coating, including the diffusion zone, from specimens which had not been exposed to conditions of elevated temperature operation in air or products of fuel combustion as would be experienced in a jet engine.

The test of example 2 was repeated on specimens of other nickel base superalloys such as those described in US. Pat. Nos. 3,155,501 issued Nov. 3, 1964, 3,642,469 issued Feb. 15, 1972 and 3,615,376 issued Oct. 26, 1971. The specimens were coated with different types of diffusion applied aluminides. However, the hydrogen pretreatment which is less desirable, both from the standpoint of high temperature requirements and lack of ability to reduce alumina, was eliminated even though hydrogen can be effective in chemically converting or reducing sulfidation reaction products from engine run components. The results showed that exposure to the mixed acid solution alone for up to about 2 hours at about 80C did not remove all of the coating. However, subsequent exposure for about onehalf hour in the alkaline permanganate solution at about 80C, followed by an additional exposure to the mixed acid solution at about 80C, with water rinses in between, removed the coatings cleanly from these specimens. The specimens were in the form of turbine blades which asLasatb smwnin aasas aa s additional steps were required after appreciable operaduring jet engine operation and disclosed that the diffusion zone remained in such rough oxided area. However, when the same method was repeated on an identical article from the same engine but with a pretreatment prior to application of the first mixed acid solution, all of the coating including the diffusion zone was removed. The pretreatment, which is in accordance with one form of the method of the present invention, is to first abrade the coating to a surface finish of no greater than rms. Greater than 100 rms provides a surface which results in subsequent imperfect coating. In this example, the same grit suspended in water was used to abrade or blast the coating surface prior to treatment with the aqueous solutions.

Thus, the present invention, in one form, provides an improved method for removing an aluminide coating from an article surface including a pretreatment involving abrading the coating surface to a certain surface finish, treating the article with a particular combination of aqueous solutions and then, optionally, abrading the treated surface to remove any chemical smut or residue. The above comparative test procedures, with and without pretreatment including grit blast, in this example with a grit of number 220, applied to different areas of the same blade, produced the same results: the coating was removed from those surfaces of the article which had been pretreated with a grit blast but was not completely removed from those areas on which no grit blast pretreatment had been applied. On such areas, 'there were remains in spots both of the difiusion zone as well as some of the originally applied coating.

An additional problem which exists in the removal of aluminide coatings is that frequently such coatings are applied to such articles as jet engine turbine blades including relatively small cooling holes or passages. In such instances, use in the above described pretreatment steps of a grit, which is not soluble in the subsequently applied liquid, can penetrate, remain in and cause blockage of such holes or passageways. The present invention, in one form, contemplates use in the pretreatment of a grit of metallic particles soluble in the subsequently applied aqueous solutions of either the mixed acid or alkaline permanganate or both. For example, during evaluation of this aspect of the present invention, various angular iron and angular steel grits were evaluated both from the standpoint of their ability to pretreat or abrade the coated surface to be stripped as well as their solubility in the subsequently applied aqueous solutions during practice of the present invention. In general, it was found that iron and steel are completely soluble in the mixed acid solution and are not as hard as alumina abrasives which can have a detrimental effect on the article. One material tested was a chilled iron grit 325 mesh having a hardness in the range of about 50 56 R In other tests, dry, angular steel abrasive in the range of 25 120 grit was used. Because steel is single phase compared with the two phase structure in cast iron, it was found that the steel was more readily soluble in the subsequently applied mixed acids solution. An added benefit from the use of such materials as angular iron abrasives is that, in addition to operating on a coating, they have a tendency to polish the substrate material.

What is claimed is:

1. In an improved method for removing an aluminide coating from an article based on an element selected from the group consisting of Co and Ni, the steps of:

providing an aqueous mixed acid solution consisting essentially of an aqueous solution of HNO in an amount greater than 25 to less than 75 volume percent nitric acid of about 70 weight percent HNO and an aqueous solution of mm, in an amount greater than 25 to less than 75 volume percent phosphoric acid of about 85 weight percent H PO providing an alkaline permanganate solution consisting essentially of, by weight, about 8 ll percent sodium hydroxide, about 8 l 1 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities;

subjecting the coating to the mixed acid solution at a temperature of about 160 180F for about A 2 hours;

rinsing the mixed acid solution from the coating;

subjecting the coating to the alkaline permanganate solution at a temperature of about 145 175F for about A 2 hours;

rinsing the alkaline permanganate solution from the 6 coating; subjecting the coating to the mixed acids solution at a temperature of about 160 180F for about /4 2 hours; and then rinsing the mixed acid solution from the coating.

2. The method of claim 1 in which:

the aqueous solution of HNO; is in an amount equal to 45 55 volume percent of the nitric acid.

3. In an improved method for removing an aluminide coating from an article based on an element selected from the group consisting of Co and Ni, the coating including products resulting from elevated temperature exposure to air and fuel combustion, the steps of:

mechanically abrading the coating with angular particles to a surface finish of no more than 100 rms; providing an aqueous mixed acid solution consisting essentially of an aqueous solution of HNO in an amount greater than 25 to less than 75 volume percent nitric acid of about weight percent HNO and an aqueous solution of l-l PO in an amount greater than 25 to less than volume percent phosphoric acid of about weight percent H PO providing an alkaline permanganate solution consisting essentially of, by weight, about 8 l 1 percent sodium hydroxide, about 8 l 1 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities;

subjecting the coating to the mixed acid solution at a temperature of about 160 l80F for about /4 2 hours;

rinsing the mixed acid solution from the coating;

subjecting the coating to the alkaline permanganate solution at a temperature of about l75F for about 2 hours;

rinsing the alkaline permanganate solution from the coating;

subjecting the coating to the mixed acids solution at a temperature of about F for about A 2 hours; and then rinsing the mixed acid solution from the coating.

4. The method of claim 3 in which the particles are of a metallic material soluble in at least one liquid to which the coating subsequently is subjected.

5. The method of claim 4 in which the material of the particles is selected from the group consisting of iron and iron base alloys.

6. The method of claim 3 including the additional subsequent step of mechanically removing chemical residue from the article. 

2. The method of claim 1 in which: the aqueous solution of HNO3 is in an amount equal to 45 - 55 volume percent of the nitric acid.
 3. In an improved method for removing an aluminide coating from an article based on an element selected from the group consisting of Co and Ni, the coating including products resulting from elevated temperature exposure to air and fuel combustion, the steps of: mechanically abrading the coating with angular particles to a surface finish of no more than 100 rms; providing an aqueous mixed acid solution consisting essentially of an aqueous solution of HNO3 in an amount greater than 25 to less than 75 volume percent nitric acid of about 70 weight percent HNO3 and an aqueous solution of H3PO4 in an amount greater than 25 to less than 75 volume percent phosphoric acid of about 85 weight percent H3PO4; providing an alkaline permanganate solution consisting essentially of, by weight, about 8 - 11 percent sodium hydroxide, about 8 - 11 percent anhydrous sodium carbonate, about 4 - 6 percent potassium permanganate, with the balance water and incidental impurities; subjecting the coating to the mixed acid solution at a temperature of about 160* - 180*F for about 1/4 - 2 hours; rinsing the mixed acid solution from the coating; subjecting the coating to the alkaline permanganate solution at a temperature of about 145* - 175*F for about 1/2 - 2 hours; rinsing the alkaline permanganate solution from the coating; subjecting the coating to the mixed acids solution at a temperature of about 160* - 180*F for about 1/4 - 2 hours; and then rinsing The mixed acid solution from the coating.
 4. The method of claim 3 in which the particles are of a metallic material soluble in at least one liquid to which the coating subsequently is subjected.
 5. The method of claim 4 in which the material of the particles is selected from the group consisting of iron and iron base alloys.
 6. The method of claim 3 including the additional subsequent step of mechanically removing chemical residue from the article. 